1. Field of the Invention
The present invention relates to a coated body for members of an electronic device, which coated body is useful as a cabinet of an electronic, electrical or optical device (which may hereinafter be represented by “electronic device”) and excellent in thermal radiation property; a coated body for members of an electronic device, which coated body is excellent in thermal radiation property and self cooling property; the above-described coated body for members of an electronic device, which coated body is also excellent in electric conductivity; parts for an electronic device excellent in such properties; a coating composition useful for the formation of a coated body excellent in thermal radiation property and electric conductivity; and an apparatus for evaluating the thermal radiation property of a coated body. Since the coated body of the present invention contains a blackening additive such as carbon black in a proper amount and has a proper coat thickness, it is excellent in thermal radiation property. It can therefore be suited for use in the products of digital information recording and/or processing products such as CD, LD, DVD, CD-ROM, CD-RAM, PDP and LCD and the products of electric, electronic and communication related fields such as personal computers, car navigation systems and car AV. Moreover, they are usable as cabinets for various electronic devices, for example, audio-video instruments such as projector, television, video and game machine, copying machine, printer, power supply box cover of air conditioner outdoor unit, control box cover, automatic vending machine and refrigerator.
The coated body of the present invention can also be used as a chromate-free coated body which does not contain harmful hexavalent chromium at all. The chromate-free coated body provided by the present invention is very useful, because it has corrosion resistance and adhesion of coating comparable to those of a chromated steal sheet and in addition, has good formability.
2. Description of the Related Art
With a recent tendency to enhanced performance and miniaturization of electronic, electrical or optical devices, heat released amount inside of the chassis of an electronic device or the like increases, causing problems such as temperature increase of the device itself (temperature increase inside of the electronic device). The temperature inside an electronic device is usually about 40 to 70° C. under normal atmospheric temperature, which happens to become about as high as 100° C. at the maximum. It has been pointed out that stable operation is disturbed by such a temperature increase exceeding the heat resistant temperature of IC, CPU (semiconductor device), disk or motor. When a temperature rises further, a semiconductor element is destroyed and gets out of order or the life of the parts of an electronic device becomes short.
As a heat releasing means for reducing the inside temperature of an electronic device (for radiating heat from an electronic device), proposed is a method of attaching a heat releasing part such as heat sink or heat pipe to the cabinet of the electronic device (to the cabinet itself, frame, shield case, back panel of liquid crystals or the like). This method is effective only for diffusing the heat released from the heat source (heating element) of an electronic device within the whole cabinet. Particularly in the case where the cabinet has a small capacity, desired heat releasing effect is not available. Moreover, it is accompanied with such inconveniences that it takes time to attach such a heat releasing part to an electronic device, an extra space is necessary for attachment, and it heightens the production cost. This method is therefore unsuited for use in electronic devices which tend to be smaller in size and lower in production cost.
Also proposed is a method of releasing heat from the cabinet of an electronic device made of a metal plate by perforating the metal plate and making use of convection caused by a fan installed to the metal plate. Since electronic devices are apt to be damaged by water or dust, such a device cannot be applied to every purpose. Moreover, it has similar problems to those of the above-described heat sink such as cost rise, and extra time and space derived from attachment of the fan.
There is accordingly an eager demand for the provision of a novel cabinet for an electronic device which can satisfy the original performances required for the electronic device (such as maintenance of air tightness in order to protect the device from water and dust, smaller size and lighter weight), while attaining a reduction in the temperature inside of the electronic device (thermal radiation property).
The cabinet of an electronic device is, in addition to the thermal radiation property, required to control a temperature rise of the cabinet itself, because this makes it possible to prevent users who touch the cabinet from getting burned during operation of the electronic device. Thus, safe products can be provided if a temperature rise of the cabinet can be suppressed. This “property to suppress a temperature rise of the cabinet itself of an electronic device” is especially called, in the present invention, “self cooling property” to distinguish it from the above-described “thermal radiation property”. The above-described heat releasing countermeasures (for example, a method of attaching a heat releasing part such as heat sink or heat pipe or a method of perforating a metal plate and installing a fan) are insufficient for obtaining a cabinet excellent in both of these properties. There is accordingly an eager demand for provision of a cabinet having both properties.
The cabinet of an electronic device is required, in addition to the above-described properties, to have excellent electric conductivity. A conventional steel plate coated into black color (a steel plate covered with a black coating) is accompanied with such a problem that an electric resistance becomes high owing to an excessively thick black coating and desired earth cannot be secured when it is applied to an electronic device.
With respect to the substrate, on the other hand, substrates are conventionally chromated in order to impart them with corrosion resistance and improve adhesion of coating, but environmental pollution brought by the use of a large amount of harmful hexavalent chromium is becoming a serious problem. There is therefore a demand for the replacement of the harmful chromate treatment with non-chromate treatment which does not contain chromium. It is however known that substrates without chromate treatment are inferior in corrosion resistance and adhesion of coating, and, formability. There is accordingly an eager demand for the provision of a chromate-free coated body usable as a cabinet for the members of an electronic device, which coated body is excellent in corrosion resistance, adhesion of coating and formability, and in addition, is excellent in thermal radiation property and self cooling property.